Method for manufacturing a thermopile on an electrically insulating substrate

ABSTRACT

Method for manufacturing a thermopile on an electrically insulating substrate. A pattern is arranged on this substrate of parts which consist of a first conductive material, to which a second conductive material is applied, and parts which consist only of the first conductive material. The second material is better electrically conducting than the first and, connected as thermopile, can thereby generate a certain thermo-voltage. The second material is applied to the first, starting from a layer of the first material on the substrate, by etching stripes therein via the so-called “blind-hole etching” technique and applying the second material in these stripes.

FIELD OF THE INVENTION

The invention relates to a method for manufacturing a thermopile on anelectrically insulating substrate, wherein the starting point is such asubstrate having thereon a layer of a first electrically conductivematerial, to which material a first sensitive lacquer coating is appliedwhich is selectively removed using a mask for placing onto this lacquercoating, whereafter at those positions where the lacquer coating isremoved a second electrically conductive material is applied to saidfirst material, wherein the second material is better electricallyconducting than the first material and the first and second material,connected as thermocouple, generate a certain thermo-voltage, theremaining part of the lacquer coating is then removed and afresh—second—lacquer coating is applied over the whole film surfacewhich, using a second mask to be laid onto this lacquer coating, isagain also selectively removed in the sense that the lacquer coatingremains intact above the second material and above determined positionsabove the first material which are determined via the mask and which donot make electrical contact with the second material, whereafter thefirst material not coated by lacquer is finally etched away and thelacquer is then removed.

BACKGROUND OF THE INVENTION

Such thermopiles are applied for instance in sensors for thermophysicalinstruments such as flow-meters, with which flow rates of gas or liquidflows—quantities of gas or liquid which flow through a conduit per unitof time—are measured.

The manufacture of thermopiles of the above stated type is described forinstance in the European patent no. 760530.

A film of a polyimide material is for instance used as substrate. Alayer of the first electrically conductive material is applied to oneside thereof. A film thus results consisting of a substrate havingthereon a first conductive material and an optionally present gluelayer.

Once the film has been degreased, a photosensitive lacquer coating isapplied to the side thereof on which the first material is situated,this in the sense that when the lacquer coating is exposed and sets atthe exposed locations, the lacquer on the unexposed locations can bewashed away—a so-called negative lacquer—or precisely the reverse: thatthe lacquer on the exposed positions can be washed away, a so-calledpositive lacquer. Exposure usually takes place via a mask with theexposure pattern laid on the first material. The lacquer is thendeveloped in the developer medium prescribed by the manufacturer of thelacquer.

The thus obtained film, which except for the parts which must be coatedwith a layer of the second material, is coated with lacquer, is nowsubjected to a galvanizing process: stripes of the second material aregrown on the uncovered first material. The height of the stripes to bethus produced is found to be limited by requirements imposed during asecond coating with lacquer, later in the manufacturing process, on thevolume of air bubbles enclosed particularly during application of thesecond material: about 12 μm above the first material was found to bethe maximum attainable.

After the stripes of the second material have been applied the remaininglacquer is removed. This takes place in an alkaline “stripper” and foras short a time as possible, in order to prevent the glue with which thefirst material is glued to the substrate from dissolving in thestripper. The subsequent application of the second lacquer coating,selective removal thereof via a second mask, etching away of theuncovered first material and, finally, removal of the lacquer also formpart of the prior art.

The limitation in the possible height of the stripes, and therewith ofthe ratio of the quantities of the second material to those of thefirst, is a great drawback of the above described known method. In thecase of a layer of the first material of 10 μm thickness and a 12 μmthick layer of the second material (the maximum thickness), said ratiothus amounts to (1:1.2). An increase in the ratio of the thickness ofthe second material to that of the first material would result in abetter functioning thermocouple.

SUMMARY OF THE INVENTION

The invention is based on the insight that it is possible using a per seknown technique, i.e. so-called “blind-hole etching”, to makethermopiles with a greatly increased ratio of the thickness of the layerof the second material to that of the first material compared with theknown thermopiles. Blind-hole etching is understood to mean etching awaymaterial to a certain depth: there remains as it were a bottom with acertain thickness.

The method according to the invention has for this purpose the featurethat after the first lacquer coating has been selectively removed, atthe positions where this has occurred the first electrically conductivematerial—preferably constantan—is etched away to a predetermined depthand only then is the second electrically conductive material—preferablycopper—applied respectively introduced at these positions. The polyimidematerial of the substrate is preferably “Kapton”.

For the materials in question the representatives thereof, i.e.constantan, copper and Kapton, will be used in each case hereinbelow.The invention is however not limited to the use of these materials.

By means of etching a type of channel is arranged in the constantan inwhich the copper is “grown”—the constantan layer thus becomes thinner atthis position and the above stated ratio of the thicknesses thus becomesmore favourable: a constantan layer of for instance 10 μm thickness canthen be etched away to a thickness of for instance 5 μm, whereafter alayer of copper of 15 μm thickness is applied. This results in a ratioof copper thickness: constantan thickness=3:1.

During etching according to the invention the etching depth is oftenfound difficult to define: a desired depth of for instance 5 μm caneasily become 4 μm or 6 μm. By starting from a thicker layer ofconstantan, which is possible when the etching technique is used,—forinstance with a thickness of 25 μm, wherein the etching depth can thenbe a maximum of 18 μm—the influence of this inaccuracy of the etchingdepth on the relevant ratio can be reduced. An associated thickness ofthe film is for instance 100 μm.

It is thus possible in theory, with a thickness of the constantan layerof 25 μm, at an etching depth of 18 μm—and therefore with a remainingconstantan thickness of 7 μm—and with a “build-up” of the copper abovethe constantan of 10 μm, to achieve a thickness ratio ofcopper:constantan=28:7=4:1. This is however found to be not whollyrealizable in practice since the etched channel does not have a whollyrectangular section perpendicularly of its length direction but isslightly sloping. This pattern is repeated in the grown copper, wherebysaid ratio will be smaller in practice.

It is recommended to first thoroughly clean the etched constantansurface prior to growing copper thereon: fine dirt particles and alsoair in the stripes can result in gaps in the copper. In a preferredembodiment the method according to the invention is thereforecharacterized in that, after etching of the constantan and prior tocopper-plating, the etched surface is cleaned by first immersing it in asolution of diluted nitric acid, whereafter it is washed with water anddried.

The copper-plating is for instance realized in an acid copper bath forseveral tens of minutes. It is found possible to thus realizethermopiles with a regularly distributed copper layer, without roughspots.

The invention comprises a thermopile manufactured using the abovedescribed method or embodiment thereof—and the invention also comprisesflow-meters equipped with a thermopile according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further elucidated with reference to the drawings,in which FIGS. 1-10 show schematically the successive steps in theprocess followed in the manufacture of a copper/copper-constantanthermopile according to the invention, starting from an electricallyinsulating substrate to which a layer of constantan is applied.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows the electrically insulating substrate 1—for instanceKapton—onto which constantan layer 2 is applied, for instance byglueing. A thickness of the constantan layer of 25 μm is found tosuffice.

FIG. 2 shows the film according to FIG. 1, but now coated with lacquercoating 3. The lacquer used is preferably a lacquer which can be set byexposure thereof and is arranged on a degreased and washed-cleanunder-layer of constantan.

FIG. 3 shows the film coated with lacquer according to FIG. 2, whereinthe lacquer is removed in accordance with a determined pattern made in amask which is placed on lacquer coating 3: when a so-called negativelacquer is used the parts 4 not exposed via the mask and thus not setare washed away.

FIG. 4 shows how at the positions where the lacquer coating is removedand the constantan thus lies uncovered, this constantan is etched awayto a certain depth. With a constantan thickness of 25 μm, etching awayto a depth of 18 μm—and therefore until a constantan layer of 7 μmremains—is possible.

Etching can take place in per se known manner with an etching agent on abasis of FeCl₃ at a temperature of about 40-50° C.

FIG. 5 shows the situation, after copper-plating, which occurs afterthe—etched—metal surface is first pre-treated respectively cleaned (forinstance immersion in a solution of nitric acid in water, followed bywashing with water and drying).

Copper-plating takes place in known manner, for instance in an acidcopper bath. The longer copper-plating continues, the thicker the copperlayer becomes. For instance: 20 minutes results in 13 μm; 30 minutesresults in 20 μm, etc.

With a film as shown in FIG. 4 a copper stripe thickness of 28 μm ispossible, which results in a ratio copper:constantan of 28:7=4:1.

FIGS. 6, 7, 8, 9 and 10 show how, using per se known process steps, thethermopile is made from the film according to FIG. 5. The lacqueradjacent to the copper stripes is first removed—FIG. 6. A fresh lacquercoating 6 is then applied over the whole surface—FIG. 7. Portions ofthis fresh lacquer coating 6 are again removed selectively—using a mask.What remains is the lacquer 7 on copper stripes 5 and protrudingslightly therefrom 8 and at positions therebetween 9 at a distance fromcopper stripes 5—FIG. 8.

FIG. 9 shows the situation after etching away of the constantan on theportions not coated with lacquer.

In FIG. 10 all lacquer has been removed with a stripper and the film isready for use as thermopile.

What is claimed is:
 1. A method for manufacturing a thermopilecomprising a plurality of thermocouples, which method comprises thesteps of: providing an electrically insulating substrate having thereona layer of a first conductive material; applying a first photosensitivelacquer coating and selectively removing the first lacquer coating usinga mask that is placed onto the first lacquer coating; etching the firstconductive material at the positions where said selective removal hasoccurred until a predetermined thickness remains, thus forming recesses;growing a second electrically conductive material on said first materialin positions where it was recessed, where the second material is abetter electrically-conductive material than the first material, whereinthe first and second materials, if connected as a thermocouple, generatea certain thermo-voltage, and wherein the thickness of the secondconductive material is greater than the remaining thickness of the firstconductive material; removing the remaining part of the first lacquercoating; applying a fresh second lacquer coating over the whole filmsurface and selectively removing the second lacquer coating using asecond mask, wherein the second lacquer coating remains intact above thesecond material and above the first material in predetermined positionswhere the first material does not make electrical contact with thesecond material, wherein said predetermined positions are determined viathe second mask, etching the first material not coated with the secondlacquer away and removing any remaining portions of the second lacquer.2. A method as claimed in claim 1, wherein the substrate consists ofKapton, the first material is constantan and the second material copper,the constantan has a thickness of 10 μm and is etched away until abottom thickness of 5 μm is reached, and the height of the copper is 15μm.
 3. A method as claimed in claim 1, wherein as starting substrate isused a film with a constantan layer with an initial thickness of morethan 10 μm, and etching takes place until a constantan thickness of 7 μmremains.
 4. A method as claimed in claim 3, wherein said initialthickness of said constantan layer is 25 μm.
 5. A method as claimed inclaim 1, wherein the ratio of the thickness of the second conductivematerial to the remaining thickness of the first conductive material isgreater than 1.2:1.
 6. A method as claimed in claim 5, wherein the ratiois 4:1 at most.
 7. A method as claimed in claim 1, wherein the thicknessof the second conductive material is greater than the depth of therecesses in the first conductive material.